Per The Unofficial Apple Weblog and The Verge, wireless carrier AT&T has indicated that all iPhone subscribers will be able to use FaceTime via a cellular connection by the end of 2013, as stated below:

“For video chat apps that come pre-loaded on devices, we currently give all OS and device makers the ability for those apps to work over cellular for our customers who are on Mobile Share or Tiered plans. Apple, Samsung and BlackBerry have chosen to enable this for their pre-loaded video chat apps. And by mid-June, we’ll have enabled those apps over cellular for our unlimited plan customers who have LTE devices from those three manufacturers. Throughout the second half of this year, we plan to enable pre-loaded video chat apps over cellular for all our customers, regardless of data plan or device; that work is expected to be complete by year end. Today, all of our customers can use any mobile video chat app that they download from the internet, such as Skype.”

AT&T was initially reluctant to enable FaceTime over cellular connection due to bandwidth concerns.

This past January, AT&T announced that it was enabling FaceTime over cellular to subscribers of its tiered data plans. Now, with its most recent announcement, iPhone users on AT&T who have grandfathered-in unlimited data plans will also be able to FaceTime to their hearts content.